1. Field of the Invention
This invention relates to coaxial cable having, as a dielectric coating on the core conductor, an extruded cellular ethylene or propylene polymer based composition.
2. Description of the Prior Art
Coaxial cables usually comprise a core conductor member coated with a dielectric, with an outer conductor member superimposed on the dielectric.
It is known that the attenuation properties of a coaxial cable can be determined by the following equation: EQU I A = 4.35 (R.sub.t /Z.sub.o) + 2.78 [(p f) F .sqroot.E]
wherein EQU R.sub.t 32 0.1*(1/d + 1/D) .sqroot.F
(for solid copper conductor) ##EQU1## EQU R.sub.t = resistance in ohms (*0.1 IS CONSTANT FOR A SOLID COPPER INNER CONDUCTOR)
A = attenuation in db per 100 foot of cable PA1 Z.sub.o = characteristic impedance of cable in ohms PA1 pf = power factor of the dielectric in radians PA1 F = test frequency in mega Hertz PA1 E = dielectric constant of the dielectric core PA1 D = outer diameter of the dielectric layer in inches PA1 d = outer diameter of the inner conductor in inches
It can be seen from this equation that Z.sub.o and R.sub.t are determined by D, d and F, all of which are fixed by the cable size and the test frequency and that the dissipation factor and dielectric constant further determine the attenuation of the cable.
The dielectric materials employed heretofore in coaxial cables were usually compositions based on ethylene polymers or on polystyrene. The dielectrics formed from the ethylene polymers, however, usually had poor electrical properties, particularly poor attenuation properties. As a result, coaxial cable made with ethylene polymer based dielectric compositions had disadvantages in that they had dissipation factors (pf) in the range of 290-500 microradians at an electrical transmission frequency range of 50-300 mega Hertz, and a dielectric constant in the range of 1.40 to 1.60. Such cables, moreover, may have to be dried, prior to the swaging on the outer conductor of the cable, to remove any water therefrom which may have been generated in the cable during the decomposition of the chemical blowing agents used to make the cellular dielectric layers used in such cables. Swaging is the process whereby the cable is passed through one or more dies to reduce the outside diameter of the outer conductor.
The dielectrics formed from polystyrene, however, while they had relatively good electrical properties, i.e. a dielectric constant of 1.1 to 1.2 and a dissipation factor in the range of 180-350 microradians, had relatively poor physical properties. As a result, coaxial cables heretofore made with polystyrene had disadvantages in that they had higher scrap rates in production and required special handling due to the poor physical properties of the surfaces of such dielectric compositions.
Prior to the present invention, therefore, it has not been readily possible to provide dielectric materials for use in coaxial cables which reproducibly provide the cable with both good mechanical properties and good electrical properties.